Lactic acid is normally prepared by the fermentation of microorganisms of the genera Lactobacillus (L. acidophilus, L. delbrueckii, L. pentosus etc.), Rhizopus, Bacillus (B. coagulans etc.), Streptococcus etc.
However, it is known that the growth, or even the viability, of the majority of these lactic acid-producing microorganisms is inhibited by the drop in the pH of the fermentation medium, this strong acidification of the medium being caused by the production of organic acids, including lactic acid itself.
It is therefore necessary to regulate the pH and it is generally acknowledged that it has to be maintained at a value of between 4 and 7, preferably of more than 4.5, for example of between 5.5 and 6.5, by the addition of bases such as alkali metal or alkaline earth metal hydroxides, or carbonates or bicarbonates.
The lactic acid is therefore present in these fermentation media in the form of salts (sodium, potassium, calcium or ammonium lactates, individually or in a mixture, depending on the base chosen for regulating the pH of the fermentation medium).
Thus all the methods of recovering the lactic acid from the fermentation media have to solve the same difficulties, i.e. separating the lactic acid salt(s), the microorganisms which produced them and the impurities in the fermentation medium (unconsumed sugars and proteins and various kinds of inorganic salts) as well as converting the lactic acid salts to lactic acid in the free form, which also requires the subsequent removal of the corresponding base generated.
Various methods have been proposed for recovering lactic acid from a fermentation medium.
It is quite generally known to purify lactic acid from lactates by the addition of sulfuric acid.
For example, if the pH of the fermentation medium is regulated with carbonates or bicarbonates, the lactic acid in the free form can be recovered by acidification of the medium with sulfuric acid.
The reaction gives rise to the formation of calcium sulfates (gypsum), which precipitate, and to the release of lactic acid in the free form, which can then be extracted with an organic solvent or adsorbed onto a suitable support and then desorbed.
However, the disadvantage of this method, which is otherwise efficient in terms of yield, is the high consumption of sulfuric acid and especially the production of large amounts of gypsum, which presents serious environmental problems in terms of waste treatment and biodegradability.
Patent JP 63/188,632 describes the regulation of the pH of the fermentation medium with zinc hydroxide to give a zinc lactate of low solubility, allowing it to be purified by crystallization.
The major disadvantage here is the need to use hydrogen sulfide, which is not easy to handle; in particular, this process gives poor crystallization yields due to excessive loss of product in the mother liquor.
Another process consists in carrying out an adsorption/desorption with a trialkylamine. For example, patent DE 2,329,480 describes a liquid-liquid extraction (or LLE) process which consists in extracting the lactic acid by means of a water-immiscible organic solvent in the presence of at least one Lewis base such as a tertiary amine. The lactic acid has to be recovered in a second liquid-liquid extraction step, in which it is transferred back into the water.
However, the disadvantage is in the formation of trialkylammonium lactates with pKa values of about 9 to 11, so the energy required to release the acid form is prohibitive.
By extension, any processes which require the decomposition of trialkylammonium salts to the corresponding acids consume large amounts of energy.
The use of NH.sub.4 OH or NaOH to regulate the pH of the fermentation medium is also described, said process making it possible to avoid the need for sulfuric acid and to recover the lactates more cleanly in the form of ammonium or sodium lactates. It is then possible to carry out an ultrafiltration step to separate said lactates from the other components of the fermentation medium, and finally to obtain the lactic acid by carrying out a separation step on ion exchange resins.
This process employs membrane and ion exchange methods by which lactic acid cannot be obtained in high purity.
Two approaches were developed to overcome this difficulty: a first approach involves chromatographic separation, especially on a strong cation exchange resin; the second involves separation by concentrating or desalinating electrodialysis and/or by bipolar fractionating electrodialysis.
According to the first approach, it was proposed to bring the medium containing e.g. sodium lactate into direct contact with a strong cation exchange resin of the hydrogen type, in the liquid form, in order to remove the sodium ions.
However, this process consumes large amounts of resins and hydrochloric or sulfuric acids to regenerate said resin, and is difficult to carry out on the industrial scale.
Patent EP 483,831 offers an alternative to this process by carrying out the separation of an organic acid (or organic acids) and inorganic salts using chromatography columns.
The process described consists in cultivating a lactic acid-producing microorganism in a fermentation medium containing glucose as the carbon source, and chromatographing said fermentation medium, containing sugars, inorganic salts and other residual impurities as well as an organic acid (or organic acids), under specific conditions.
More precisely, the process consists in passing said starting fermentation medium, containing the organic acid (or organic acids), through a column containing the cation exchange resin, resulting in the retention of said organic acid (or said acids) on the column. An eluent of the mineral acid type is then passed through in order to desorb any organic acid. In the case where it is desired to separate several organic acids (such as tartaric acid and gluconic acid), an additional step consists in separating the acids from the resulting eluate.
The description states that it is absolutely essential for the pH to be below the pKa of the acid in question.
More particularly, an example is given of a process for the preparation of lactic acid from an L. delbrueckii fermentation medium, the pH of the medium being regulated to 5.8 with NaOH.
The microorganisms are removed from said medium, containing 71 g/l of sodium lactate, and the medium is then acidified to pH 1.6 with sulfuiric acid. Said acidified solution is then passed through a chromatography column in which the cationic resin has first been equilibrated to pH 3.09 by the passage of dilute sulfuric acid. The column is then eluted with more sulfuric acid, still at a pH below the pKa of lactic acid. i.e. below 3.87, making it possible to recover one fraction containing sodium sulfate and another fraction containing lactic acid.
However, this process again requires the use of large amounts of sulfuric acid, both to lower the pH of the fermentation medium and maintain the pH in the chromatography column, and also to adjust the pH of the eluent to a value below the pKa of lactic acid.
The more it is desired to lower the pH, so as not only to limit the proportion of lactic acid salts in equilibrium with the free lactic acid formed, but also to limit the loss of free lactic acid with the impurities (residual sugars, proteins and inorganic salts in the fermentation medium) which are normally eluted in the first fractions of the chromatographic separation, as furthermore deplored in patent EP 483,831, the larger these amounts of sulfuric acid will be.
According to the second approach, the preferred procedure is to carry out a separation by electrodialysis directly on the fermentation medium containing the lactates.
Thus patent EP 393,818 describes a process wherein the fermentation medium, from which the microorganisms have been removed, is subjected to a first, conventional electrodialysis in order to recover and concentrate the lactic acid salts in aqueous solution with the nitrogenous impurities removed, and said aqueous solution is then subjected to a second electrodialysis, this time for fractionation, in order to recover the free lactic acid and the corresponding base. The solution containing the free lactic acid then has to be treated on a strong ion exchanger in the acid form to remove the sodium ions or other cations, and the resulting solution then has to be treated on a weakly basic ion exchanger to remove all traces of sulfuric acid, sulfates or other impurities to give a purified lactic acid.
The prime disadvantages of this process are the energy cost of this system, which involves two electrodialyses in series, and the size of the volumes handled. For example, the energy cost is directly linked to the need to concentrate the solutions containing the lactic acid salts in order to perform the fractionating electrodialysis, these costs increasing proportionately to the amounts of high-purity lactic acid which it is desired to recover.
The second disadvantage relates to the phenomenon of fouling of the electrodialysis membranes.
Thus patent U.S. Pat. No. 5,681,728 insists on the need to carry out a nanofiltration step and/or to use a chelating resin as an obligatory first step in order for the concentrating or fractionating electrodialyzers to function correctly; otherwise the cost of purifying the lactic acid increases proportionately to the need to clean or replace the electrodialysis membranes.
Patent U.S. Pat. No. 5,814,498 also describes the implementation of a nanofiltration step on the clarified fermentation medium (i.e. the fermentation medium from which the lactic acid-producing microorganisms have been removed), prior to the use of bipolar fractionating electrodialysis, in order to remove the divalent cations from said fermentation medium, said cations reducing the efficiency of the electrodialysis.
Likewise, patent CA 2,227,398 describes the implementation of a nanofiltration step or a basic loop, consisting of a column of cationic resin, in order to remove the polyvalent ions before carrying out the actual electrodialysis with anionic or cationic membranes.
However, the processes improved in this way still all suffer in practice from the succession of numerous cumbersome steps which make the purification of the lactic acid from the fermentation medium particularly long and tiresome.